The present invention relates to an arrangement for controlling a work vehicle, comprising a power source, and a hydraulic circuit comprising a pump driven by the power source, at least one hydraulic actuator arranged in fluid connection with the pump via a first conduit, and a variable displacement hydraulic motor unit arranged in fluid connection with the actuator and downstream the actuator via a second conduit. The invention is also related to a method for controlling a work vehicle.
The pump is normally operatively driven by an internal combustion engine arranged for propelling the work vehicle.
The term work vehicle comprises different types of material handling vehicles like construction machines, such as a wheel loader, a backhoe loader, a motor grader and an excavator. The invention will be described below in a case in which it is applied in a wheel loader. This is to be regarded only as an example of a preferred application.
Said actuator may be a linear actuator in the form of a hydraulic cylinder. A wheel loader comprises several such hydraulic cylinders in order to perform certain work functions. A first pair of hydraulic cylinders is arranged for turning (steering) the wheel loader. A second pair of hydraulic cylinders is arranged for lifting a load arm unit and a further hydraulic cylinder is arranged on the load arm unit for tilting an implement, for example a bucket or forks, arranged on the load arm unit.
Conventional hydraulic systems normally comprise a directional valve arranged upstream of the hydraulic actuator for controlling the supply of fluid from the pump to the actuator and thereby also the movement of the actuator. The directional valve is adjusted in a continuously variable way according to a desired movement of the implement. Thus, the fluid flow from the pump is throttled to a greater or lesser extent in order to achieve the desired movement.
Prior art hydraulic systems have some energy losses during operation. Some of these energy losses are described below.
For example, when a function is actuated, the load is brought to a certain speed (for example during steering of the vehicle). Braking the load to a lower speed or to a stop is done by throttling the fluid. The kinetic energy from the load is thereby transmitted to the fluid via the valve outlet.
Further, there is a risk of vehicle instability in certain situations. For example, when the vehicle is steered by means of the associated actuators, the vehicle may bounce sideways.
Further, during a lifting operation of the implement, it is first raised to a certain level by supplying the associated actuators with hydraulic energy. This energy is transferred to potential energy when the implement is in the raised position. This energy is throttled via said valve when the implement is lowered. The loss of energy is particularly high when a load is lowered (for example when a pallet is lowered from a rack).
Further, during a tilting operation of the implement (in the form of a bucket) , it is first tilted upwards to a certain level by supplying the associated actuator with hydraulic energy. This energy is transferred to potential energy when the implement is in the raised position. This energy is throttled via said valve when the implement is tilted downwards again.
Further, when the implement is lowered and when the implement (bucket) is emptied, respectively, the gravity acts as a downward force. The pump continue to pump also in this situation, when only the gravity force in principle could be used to move the implement.
It is known to use a so-called Load Sensing hydraulic system (LS system) in the work vehicle. The LS system comprises means for sensing a load pressure subjected to the actuator during operation. More specifically, the load is sensed and the output pressure of the pump is controlled so that it exceeds the load pressure existing in the actuator by a predetermined differential. More specifically, the pressure (an LS signal) from the actuator for the load may be sensed via a shuttle valve and via an activated control valve unit associated with the actuator for the load. The pump then delivers a hydraulic fluid flow to the actuator, the level of which depends on the extent to which the activated control valve unit is operated.
The LS system generally has a relatively high efficiency. However, the LS-system has some energy losses. Some of these energy losses are described below.
The pump in a conventional load-sensing system works for keeping a constant pressure drop over the directional valve. The flow is determined by the opening area of the valve. The magnitude of the pressure drop depends on the system design and valve type, but is normally 10-25 Bar. A wheel loader is often operated with a low number of revolutions of the engine and several work functions are performed at the same time. This leads to that the pump is unable to saturate the pressure of the valves when they are fully opened, which in turn leads to lower pressure drops.
When several work functions are actuated at the same time in a LS-system with a common pump, the pump needs to generate a pressure level that can handle the highest actuator pressure. This means that the valves controlling the further actuator(s) (functions), will get very high pressure drop, which will be throttled away in the associated valve.
Oil resources are becoming more scarce in the world, which increases the prices of oil-based fuels. The efficiency of vehicles requiring oil-based fuels therefore becomes more important in the future. For work vehicles, there is a problem of energy losses in the hydraulic systems.
In U.S. Pat. No. 6,789,387, a hydraulic system for recovering energy in a work vehicle is disclosed. The system is arranged to recover energy during an overrunning load condition, i.e when a hydraulic cylinder is retracted due to its own weight after it has been extended to lift a load. An overrunning load condition is sensed and a valve is thereafter actuated so that a fluid from the cylinder is directed to a hydraulic motor for producing a torque output. One disadvantage is that the system is limited to recover energy only during said overrunning condition.
In U.S. Pat. No. 6,725,581, a hydraulic system for recovering energy in a work vehicle is disclosed. The system comprises several hydraulic actuators for performing different work functions. Several switches are arranged for guiding a return oil from one of said hydraulic actuators depending on a detected back pressure of the actuators. A pump motor is rotatably driven by the return oil from the selected hydraulic actuator. A dynamo-electric generator is coupled to the pump motor for generating electric power from the rotary force of the pump motor. One disadvantage is that the system is limited to recover energy for only one work function at the same time.
It is desirable to create conditions for a system that is more energy-efficient than previously known systems and solves or at least relieves some of the problems discussed above.
According to an aspect of the present invention, the means for controlling movement of the actuator is formed by the variable displacement hydraulic motor unit. Thus, the fluid is pumped in the hydraulic circuit from the pump to the actuator via the first conduit and in return from the actuator to the hydraulic motor unit via the second conduit and further to a reservoir. The wording “movement of the actuator” refers in this case to the speed of the actuator.
Thus, the hydraulic motor unit, which is arranged downstream of the actuator, is used for controlling the movement of the actuator. Hence, no directional control valve is required upstream of the actuator for controlling the actuator and the abovementioned problems with throttling losses are eliminated. Further, the variable displacement hydraulic motor unit is preferably the only means for controlling movement of the actuator.
Thus, the means for controlling movement of the actuator is formed by the variable displacement hydraulic motor unit in combination with that the fluid connection through the first conduit from the pump to the actuator is free from actuator movement controlling throttling means.
According to an aspect of the invention, the fluid is directly supplied from the pump to the actuator. In other words, the fluid connection through the first conduit from the pump to the actuator is free from throttling means, i.e the first conduit is fully open and the fluid flow is supplied to the actuator is a non-manipulated, non- throttled manner.
According to an aspect of the invention, the arrangement comprises means for electrically controlling the displacement of the variable displacement motor unit. Said electrical control means is preferably formed by a controller.
The control of the displacement may be done in response to receiving a work function signal from an operator maneuverable control lever. The signal from the operator lever may further be manipulated in the controller. For example, ramps for initiating and terminating an actuator movement, respectively, may be stored in a memory and used for displacement control. The displacement of the motor unit may also be regulated according to a sensed operating parameter of the vehicle, such as the number of revolutions of the power source. The movement of the actuator may thereby be controlled in an effective and smooth way.
According to a further aspect of the invention, the motor unit is arranged for a rotation connection to the power source in order to transmit energy to the power source.
By virtue of this arrangement, any excess hydraulic energy supplied by the pump is recovered back to the power source via the hydraulic motor unit. Excess hydraulic energy is supplied by the pump when it is working at an unnecessary high pressure level (which is the case for example in a system with a constant pump pressure).
Further, a potential energy achieved when the implement is raised to a raised position is recovered by the motor unit and transmitted to the power source when the implement is lowered. The recovery of energy is particularly high when a load is lowered (for example when a pallet is lowered from a rack).
Since the hydraulic motor unit is connected to the power source such that it transmits energy from the fluid flow to the power source, the problem of energy losses in the conventional directional valve is solved and any excess hydraulic energy provided by the pump may be recovered in the hydraulic motor unit.
According to a further aspect of the invention, the arrangement comprises a set of on/off valves arranged on the first and second conduit for actuating the associated hydraulic actuator. Thus, these on/off valves are adapted to be arranged in one of two end positions; a first position, in which the fluid connection is fully open and a second position, in which the fluid connection is fully closed. The above mentioned problem with pressure drop is thereby substantially solved. The movement of the on/off valve from one end position to the other end position may be controlled in a continuous way so that the transition is not too abrupt. For example, the first and last part of the movement distance may comprise a ramp for a smooth operation.
In the case of a hydraulic cylinder for controlling a work function, there are two input conduits to and two output conduits from the cylinder. A first input conduit is connected to a piston side and a first output conduit is connected to a piston rod side. A second input conduit is connected to the piston rod side and a second output conduit is connected to the piston side. An on/off valve is arranged on each of these four input/output conduits and by simultaneously open the on/off valves at the first conduits or the second conduits, the cylinder can be moved in different directions by means of the pressurized fluid from the pump. Preferably, the controller is arranged for electrically controlling the on/off valves based on operator command signals.
According to a further aspect of the invention, the arrangement comprises means for sensing a load pressure subjected to the actuator during operation. By using the load-sensing system in the control arrangement according to the invention, several energy losses associated with hydraulic systems with conventional control of the actuator (via a directional valve upstream of the actuator) may be relieved.
According to a further aspect of the invention, the arrangement comprises a plurality of hydraulic actuators for performing a plurality of work functions, and that one variable displacement motor unit is arranged for controlling each work function. Thus, each work function, like steering, lift and tilt is connected to a separate motor unit. In this way, the movement of each actuator may be controlled independently from the other actuators. The recovery of energy is especially efficient when several work functions are used simultaneously. The pump supplies a sufficiently high pressure for the highest loaded work function and all excess energy is recovered via the motor units.
According to a further aspect of the invention, the power source is connected in such a way to at least one further energy using system/component in the vehicle that energy recovered by the motor unit may be transmitted to it. For example, when the implement is lowered, the energy recovered by the motor unit is larger than the -energy supplied by the pump due to the fact that the motor unit will receive the potential energy of the load arm unit and the load. This excess energy can be used by the power source to drive for example the vehicle driveline and/or further vehicle systems like the service brake system and/or components like fans and generators etc.
The term “driveline” is in the following referred to as the arrangement downstream the engine for transmitting power from the engine to the vehicle ground engaging members (wheels or tracks).
It is also desirable to achieve a control method that is more energy-efficient than previously known methods and solves or at least relieves some of the problems discussed above.
A method according to An aspect of the present invention is also disclosed.
Further advantageous embodiments and further advantages of the invention emerge from the detailed description below.